The present invention relates to a sheet feeding apparatus for separating sheets stacked on a stacker into a single sheet and feeding the sheet to a processing position for reading an image on the sheet or printing the sheet. More particularly, the present invention relates to a sheet feeding apparatus provided with an ultrasonic wave sensor for detecting a sheet supplied to a processing position from a stacker or detecting a double feed of two or more sheets, and an image reading apparatus equipped with the same.
In an image reading apparatus such as a scanner or an image forming apparatus such as a printer, a sheet stacked on a stacker is picked up and transported to a predetermined processing position one at a time. The sheet is processed on a platen disposed at the processing position. In such an apparatus, it is necessary to accurately transport the sheet from the stacker to the platen in view of precise processing. Therefore, sensors are arranged along a sheet transport path for monitoring the sheet from the stacker to the processing position. The sensors include various types for detecting timing when a leading edge of the sheet reaches a specific position, timing when a trailing edge of the sheet passes, or detecting whether two or more sheets are transported, so that the sheet is properly transported thereafter.
Japanese Patent Publication (Kokai) No. 10-257595 has disclosed an ultrasonic sensor for detecting a transport process of a sheet. In a conventional ultrasonic wave sensor, a piezoelectric diaphragm such as a piezoelectric ceramic is disposed on a wave sending side (transmission side). A pulse voltage with a specific frequency is applied to the piezoelectric diaphragm, so that the piezoelectric diaphragm vibrates to generate an ultrasonic wave. A similar piezoelectric diaphragm is disposed at an opposing position across the sheet as a wave receiving side (reception side) for receiving the ultrasonic wave from the wave sending side to convert a vibration into an electrical signal. The electrical energy applied to the piezoelectric diaphragm at the wave sending side (wave sending element) is compared with the electrical energy generated in the piezoelectric diaphragm at the wave receiving side (wave receiving element), so that it is determined whether the sheet exists, or a plurality of sheets is overlapped.
In order to detect an overlapped state of sheets with such an ultrasonic wave sensor, it is necessary to accurately detect ultrasonic wave energy (output from a wave receiving element as electrical energy) attenuating through the sheets between the wave sending element and the wave receiving element. Further, in order to prevent an ultrasonic wave sent from the wave sending element from reflecting on the sheet and returning to the wave sending element to interfere with an incoming ultrasonic wave, U.S. Pat. No. 6,212,130 has disclosed a technique in which a wave sending element and a wave receiving element are inclined with a specific angle relative to a sheet.
Furthermore, Japanese Utility Model (Kokai) No. 06-49567 has disclosed a technique in which a wave sending element and a wave receiving element are disposed between front and back rollers arranged with a specific distance therebetween for detecting a sheet in a state that the sheet is transported linearly. That is, the front and back rollers nip and transport the sheet linearly when the sensor detects the sheet. Accordingly, it is possible to accurately detect the sheet even when a leading edge or a trailing edge of the sheet is bent or vibrates up and down. In order to detect a difference between a single sheet and a plurality of sheets with the ultrasonic wave or an amount of light transmitting through the sheet moving at a specific speed, it is necessary to reduce variation of the sheet and measure a predetermined length (area) of the sheet for smoothing.
As described above, in order to detect the sheet or the double feed of the sheets with an attenuation amount of the ultrasonic energy of the ultrasonic wave passing through the sheet, it is necessary to maintain the ultrasonic wave constant between the wave sending element and the wave receiving element. In this case, if dust such as paper dust is accumulated on a surface of the wave sending element or the wave receiving element, the ultrasonic wave may falsely attenuate due to the dust, thereby causing erroneous detection. In particular, when a plurality of sheets having different paper qualities and thicknesses is detected, even a small variation due to the dust may have a significant effect on the detection.
In view of the problems described above, an object of the present invention is to provide a sheet feeding apparatus having a simple structure with low cost in which an ultrasonic wave sensor can detect a sheet transported from a stacker without a large influence due to dust such as paper dust. Accordingly, it is possible to maintain detection accuracy regardless of environment or duration of use.
Further objects and advantages of the invention will be apparent from the following description of the invention.